This invention relates generally to a system for inspecting printed circuit boards, and more particularly, to a system for inspecting printed circuit boards which may be warped to varying extent to accurately verify correct component placement and proper solder connections.
As is known to persons skilled in the art, a printed circuit board is used for mounting and electrically interconnecting electrical components in a predetermined manner Traditionally, this was accomplished using through-the-hole technology which involved the placement of components (both passive and active) on the top side of a printed circuit board so that the connecting leads or pins of the components extended through holes (generally plated) provided in the circuit board. The leads or pins extending through the circuit board were then bent over to mechanically clamp the electrical components to the printed circuit board, followed by an appropriate soldering operation to complete the necessary connections. More recently, this has been accomplished using surface mount technology (SMT) which involves the placement of components (both passive and active) on top of the printed circuit board so that their connecting leads communicate with conductive pads associated with the top face of the printed circuit board. The components are then electrically connected to their associated pads by an appropriate soldering operation, from the top of the board.
In either case, it has become traditional for such printed circuit boards to be constructed mechanically, using automated assembly machines which operate to reduce the often prohibitive costs of manually assembling a printed circuit board. While reducing overall costs, such automated assembly has been found to result in periodic mis-insertions (through-the-hole) or misplacements (SMT) of the components, and their connecting leads or pins, resulting in an ineffective or unreliable electrical connection. Considering the costs which inherently result from such assembly errors, a variety of steps were taken to locate potential errors as early as possible in the assembly process This is because the cost of correcting an error increases significantly as the board proceeds through subsequent manufacturing, and distribution steps. For example, an assembly error which is found prior to soldering is inexpensively repaired, while an assembly error which is not found until final assembly (in a product) is often so costly to repair that the assembled board is often discarded rather than attempting to locate and correct the problem.
Early efforts to locate such assembly errors involved a visual inspection of each printed circuit board at a desired stage of the manufacturing process, by human operators using the naked eye, or possibly a stereo microscope or the like. However, since it is not uncommon for a typical printed circuit board to have from 1,000 to 10,000 leads or pins for connection, such a job was found to be extremely tedious and inaccurate. Accordingly, even under the best of conditions, a significant number of missed assembly errors was found to result. Added to this were the increased costs of such an inspection process due to the significant amount of time which was required, and the correspondingly increased inventories which were necessary to accommodate such visual inspections.
For this reason, steps were taken to develop automated systems for inspecting printed circuit boards, to replace such visual inspections. Two such devices, which have found wide acceptance in the industry, are the Model 5511A and the Model 5512A Printed Circuit Board Inspection Systems which are distributed by the Universal Instruments Corporation of Binghamton, N.Y. These devices generally employ a series of cameras which are mounted within a fixture (an inspection head) adapted for movement upon an X-Y table to inspect various portions of a printed circuit board (either from its underside for a through-the-hole board or from its top for an SMT board) which is received by the X-Y table. The inspection head is sequentially advanced to successive viewing fields (typically 2".times.2" for through-the-hole boards and 1".times.1" for SMT boards) established along the surface of the printed circuit board to inspect (through microprocessor analysis) the exposed components, and their connecting leads or pins, to verify their effective placement based upon a comparison with pre-established norms for the particular printed circuit board which is being tested Any defects are then reported to the operator, for appropriate correction.
The accuracy of this inspection is enhanced by providing an inspection head which incorporates a series of four angled, orthogonally placed cameras, each of which is provided with its own corresponding light source (preferably a series of controllable LED's). Such structure is provided to enable each of the series of viewing fields defined along the surface of a printed circuit board to be inspected from four different perspectives, making sure that each component feature, connecting lead or pin is detected in at least one of these four orientations. Through microprocessor controls associated with the apparatus, each of a series of anticipated components and their connecting leads or pins can be checked for proper placement in a highly reliable and automated fashion, eliminating the need for tedious visual inspections and the like.
However, it was found that the same angled camera placement which led to an enhanced accuracy of the inspection procedure also led to certain complications in cases where the printed circuit boards being inspected were warped to some extent. At the center of this problem was that the successive viewing fields of a warped printed circuit board will tend to be positioned differently relative to the inspection head and the series of cameras which it contains, which in turn tends to cause an apparent lateral shift between the viewed (by the cameras) position of a particular component and its connecting leads or pins, and the location of an associated inspection region which has been established for that component feature, connecting lead or pin based upon certain pre-established norms for the printed circuit board which is being inspected (i.e., a parallax). What is more, the degree of this lateral shifting tended to vary from one viewing field to another, depending upon the local curvature of the printed circuit board and the resulting distance between a particular viewing field of the printed circuit board and the series of cameras associated with the inspection head.
For this reason, it became necessary to compensate for the potentially adverse affects of a warped printed circuit board on the board inspection procedure. This was accomplished by preceding the inspection of a particular viewing field with a searching procedure in which a small number of selected (empirically selected) component features, connecting leads or pins were actively searched for and located, followed by a comparison with their anticipated placements. Any resulting deviations (resulting from the curvature of a warped board) were then used to correspondingly alter the anticipated locations of any inspection regions associated with the viewing field being inspected, to effectively realign the affected inspection regions for use in connection with the inspection procedure which was to follow.
While providing adequate compensation for warped printed circuit boards in many cases, certain difficulties were experienced. For example, if one of the selected component features, connecting leads or pins was not placed on the printed circuit board (an assembly error), or was not correctly located on the printed circuit board (an alignment error), an erroneous compensation could result which would render the subsequent inspection procedure ineffective. It was also possible to mistake an adjacent feature (particularly on a densely populated board) for the feature which was being searched for, again leading to an erroneous compensation and an ineffective inspection procedure.
It therefore became desirable to develop a system for more accurately inspecting potentially warped printed circuit boards, by more accurately compensating for apparent lateral shifting of the viewed image resulting from the curvature of a warped printed circuit board.